Chemistry for Biologists: Photosynthesis Leaves and leaf structure | The structure of the chloroplast and photosynthetic membranes | Stages of photosynthesis | Non-cyclic phosphorylation (the Z scheme) | Chemiosmosis and ATP synthesis | Cyclic phosphorylation | The light-independent reactions | Summary of stages of photosynthesis | Factors affecting the rate of photosynthesis | Test your knowledge Photosynthesis Photosynthesis is the process by which plants, some bacteria and some protistans use the energy from sunlight to produce glucose from carbon dioxide and water. This glucose can be converted into pyruvate which releases adenosine triphosphate (ATP) by cellular respiration. Oxygen is also formed. Photosynthesis may be summarised by the word equation: carbon dioxide + water glucose + oxygen The conversion of usable sunlight energy into chemical energy is associated with the action of the green pigment chlorophyll. Chlorophyll is a complex molecule. Several modifications of chlorophyll occur among plants and other photosynthetic organisms. All photosynthetic organisms have chlorophyll a. Accessory pigments absorb energy that chlorophyll a does not absorb.

Accessory pigments include chlorophyll b (also c, d, and e in algae and protistans), xanthophylls, and carotenoids (such as beta-carotene). Chlorophyll a absorbs its energy from the violet-blue and reddish orange-red wavelengths, and little from the intermediate (green-yellow-orange) wavelengths. Chlorophyll - click on image to open All chlorophylls have: a lipid-soluble hydrocarbon tail (C20H39 -) a flat hydrophilic head with a magnesium ion at its centre; different chlorophylls have different side-groups on the head The tail and head are linked by an ester bond. Leaves and leaf structure Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). A leaf may be viewed as a solar collector crammed full of photosynthetic cells. The raw materials of photosynthesis, water and carbon dioxide, enter the cells of the leaf, and the products of photosynthesis, sugar and oxygen, leave the leaf. Water enters the root and is transported up to the leaves through specialized plant cells known as xylem vessels.

Land plants must guard against drying out and so have evolved specialized structures known as stomata to allow gas to enter and leave the leaf. Carbon dioxide cannot pass through the protective waxy layer covering the leaf (cuticle), but it can enter the leaf through the stoma (the singular of stomata), flanked by two guard cells. Likewise, oxygen produced during photosynthesis can only pass out of the leaf through the opened stomata. Unfortunately for the plant, while these gases are moving between the inside and outside of the leaf, a great deal of water is also lost. Cottonwood trees, for example, will lose 100 gallons (about 450 dm3) of water per hour during hot desert days. The structure of the chloroplast and photosynthetic membranes The thylakoid is the structural unit of photosynthesis. Both photosynthetic prokaryotes and eukaryotes have these flattened sacs/vesicles containing photosynthetic chemicals. Only eukaryotes have chloroplasts with a surrounding membrane. Thylakoids are stacked like pancakes in stacks known collectively as grana. The areas between grana are referred to as stroma.

While the mitochondrion has two membrane systems, the chloroplast has three, forming three compartments. Structure of a chloroplast Stages of photosynthesis When chlorophyll a absorbs light energy, an electron gains energy and is 'excited'. The excited electron is transferred to another molecule (called a primary electron acceptor). The chlorophyll molecule is oxidized (loss of electron) and has a positive charge. Photoactivation of chlorophyll a results in the splitting of water molecules and the transfer of energy to ATP and reduced nicotinamide adenine dinucleotide phosphate (NADP). The chemical reactions involved include: condensation reactions - responsible for water molecules splitting out, including phosphorylation (the addition of a phosphate group to an organic compound) oxidation/reduction (redox) reactions involving electron transfer Photosynthesis is a two stage process.

The Light dependent reactions, a light-dependent series of reactions which occur in the grana, and require the direct energy of light to make energy-carrier molecules that are used in the second process: light energy is trapped by chlorophyll to make ATP (photophosphorylation) at the same time water is split into oxygen, hydrogen ions and free electrons: 2H2O 4H+ + O2 + 4e- (photolysis) the electrons then react with a carrier molecule nicotinamide adenine dinucleotide phosphate (NADP), changing it from its oxidised state (NADP+) to its reduced state (NADPH): NADP+ + 2e- + 2H+ NADPH + H+ The light-independent reactions, a light-independent series of reactions which occur in the stroma of the chloroplasts, when the products of the light reaction, ATP and NADPH, are used to make carbohydrates from carbon dioxide (reduction); initially glyceraldehyde 3-phosphate (a 3-carbon atom molecule) is formed.

The light-dependent reactions When light energy is absorbed by a chlorophyll molecule its electrons gain energy and move to higher energy levels in the molecule (photoexcitation). Sufficient energy ionises the molecule, with the electron being 'freed' leaving a positively charged chlorophyll ion. This is called photoionisation. In whole chloroplasts each chlorophyll molecule is associated with an electron acceptor and an electron donor. These three molecules make up the core of a photosystem. Two electrons from a photoionised chlorophyll molecule are transferred to the electron acceptor. The positively charged chlorophyll ion then takes a pair of electrons from a neighbouring electron donor such as water. An electron transfer system (a series of chemical reactions) carries the two electrons to and fro across the thylakoid membrane. The energy to drive these processes comes from two photosystems: Photosystem II (PSII) (P680) Photosystem I (PSI) (P700) It may seem confusing, but PSII occurs before PSI. It is named because it was the second to be discovered and hence named second. The energy changes accompanying the two sets of changes make a Z shape when drawn out.

This is why the electron transfer process is sometimes called the Z scheme. Key to the scheme is that sufficient energy is released during electron transfer to enable ATP to be made from ADP and phosphate. Chemiosmosis and ATP synthesis The components of non-cyclic phosphorylation are found in the thylakoid membranes of the chloroplast. Electrons passing through the transport chain provide energy to pump H+ ions from the stroma, across the thylakoid membrane into the thylakoid compartment. H+ ions are more concentrated in the thylakoid compartment than in the stroma. We say there is an electrochemical gradient. H+ ions diffuse from the high to the low regions of concentration. This drives the production of ATP. Cyclic phosphorylation The net effect of non-cyclic phosphorylation is to pass electrons from water to NADP. Energy released enables the production of ATP. But much more ATP is needed to drive the light-independent reactions. This extra energy is obtained from cyclic phosphorylation. This involves only Photosystem I which generates excited electrons. These are transferred to the electron transport chain between PSII and PSI, rather than to NADP+ and so no NADPH is formed.

The cycle is completed by electrons being transported back to PSI by the electron transport system. The light-independent reactions In the Light-Independent Process (the Dark reaction) carbon dioxide from the atmosphere (or water for aquatic/marine organisms) is captured and modified by the addition of hydrogen to form carbohydrates. The incorporation of carbon dioxide into organic compounds is known as carbon fixation. The energy for this comes from the first phase of the photosynthetic process. Living systems cannot directly utilize light energy, but can, through a complicated series of reactions, convert it into C-C bond energy that can be released by glycolysis and other metabolic processes. Carbon dioxide combines with a five-carbon sugar, ribulose 1,5-biphosphate (RuBP). A six-carbon sugar forms but is unstable. Each molecule breaks down to form two glycerate 3-phosphate (GP) molecules. These glycerate 3-phosphate (GP) molecules are phosphorylated by ATP into glycerate diphosphate molecules. These are reduced by NADPH to two molecules of glyceraldehyde 3-phosphate (GALP).

Of each pair of GALP molecules produced: one molecule is the initial end product of photosynthesis; it is quickly converted to glucose and other carbohydrates, lipids or amino acids one molecule forms RuBP through a series of chemical reactions The first steps in the Calvin cycle The first stable product of the Calvin Cycle is phosphoglycerate (PGA), a 3-C chemical. The energy from ATP and NADPH energy carriers generated by the photosystems is used to phosphorylate the PGA. Eventually there are 12 molecules of glyceraldehyde phosphate (also known as phosphoglyceraldehyde or PGAL, a 3-C), two of which are removed from the cycle to make a glucose. The remaining PGAL molecules are converted by ATP energy to reform six RuBP molecules, and thus start the cycle again. Factors affecting the rate of photosynthesis The main factors are light intensity, carbon dioxide concentration and temperature, known as limiting factors. As light intensity increases, the rate of the light-dependent reaction, and therefore photosynthesis generally, increases proportionately. As light intensity is increased however, the rate of photosynthesis is eventually limited by some other factor. Chlorophyll a is used in both photosystems. The wavelength of light is also important.

PSI absorbs energy most efficiently at 700 nm and PSII at 680 nm. Light with a high proportion of energy concentrated in these wavelengths will produce a high rate of photosynthesis. An increase in the carbon dioxide concentration increases the rate at which carbon is incorporated into carbohydrate in the light-independent reaction and so the rate of photosynthesis generally increases until limited by another factor. Photosynthesis is dependent on temperature. It is a reaction catalysed by enzymes. As the enzymes approach their optimum temperatures the overall rate increases. Above the optimum temperature the rate begins to decrease until it stops. BASIC PHOTOSYNTHESIS BASIC PHOTOSYNTHESIS Dickinson College, Carlisle, PA 17013 brennant@dickinson.edu Introduction The functioning of the Earth's ecosystem is dependent upon a continuous input of light energy in the form of photons coming from the sun. The process by which light energy is captured and incorporated into molecules that can be used by living organisms is called photosynthesis. It takes place in green plants, algae, and some kinds of bacteria.

Organisms can be grouped into two major categories: autotrophs, which carry on photosynthesis and manufacture their own food, and heterotrophs, which do not perform photosynthesis themselves but must consume autotrophs or other organisms that ate autotrophs. In other words, photosynthetic organisms are at the base of the food chain, and with only a very few exceptions all organisms on Earth depend upon photosynthesis as the source of their food and energy. Although photosynthesis is a very complex process, it can be divided into two parts. In the first part, light energy is captured and used to make high energy chemical molecules. In the second part, those high energy molecules are used to convert atmospheric carbon dioxide (CO2) into carbohydrates and other energy containing compounds. These then serve as food for the photosynthetic plant itself or for an animal that eats the plant. During this process, water molecules (H2O) are split apart and the oxygen from them is released into the atmosphere as oxygen gas (O2). This is an important byproduct of photosynthesis because most organisms have evolved ways of using O2 to help them process their foods more efficiently.

Some of the O2 also moves into the upper atmosphere where it forms ozone (O3), which is important in protecting the Earth's surface from ultraviolet radiation. We will now examine these and other aspects of photosynthesis in more detail. Capture of Light Energy and Formation of High Energy Molecules This first part of photosynthesis is sometimes referred to simply as the "light reactions." In essence it is an energy conversion process, i.e. the conversion of light energy into chemical energy. This discussion will be based upon the way the process is believed to function in higher plants, although the fundamental mechanisms are generally similar in other types of organisms. Photosynthetic Pigments and the Structure of the Leaf The first law of photochemistry and photobiology tells us that in order for light to have an effect in a biological system it must first be absorbed. Pigments are molecules that are specialized for the absorption of light, and they therefore must be found in all photosynthetic organisms. Two major classes of photosynthetic pigments that occur in higher plants are the chlorophylls and the carotenoids.

Chlorophylls consist of a light-absorbing porphyrin ring with a magnesium atom at the center and a long phytol tail that anchors the molecule in a membrane (Figure 1). They absorb light in the blue and red parts of the spectrum, but the green wavelengths are transmitted or reflected. This is what makes leaves appear green in color. Chlorophylls a and b both occur in higher plants. They have slight differences in structure and absorbance spectra. Protochlorophyll is a biosynthetic precursor of chlorophyll. Figure 1. Chlorophyll Structures. The structural differences from chlorophyll a are spotlighted for each selection. Carotenoids (Figure 2) are yellow to orange in color. They extend the range of wavelengths that can be utilized in photosynthesis by absorbing some light in the blue region of the spectrum that chlorophyll does not absorb. They also participate in photosynthesis by protecting the chlorophyll from reactive oxygen compounds. Carotenoids are responsible for the brilliant colors of autumn leaves. They are present in leaves all the time, but remain unseen until the larger quantities of chlorophyll begin to break down with the onset of short days and cool weather.

The carotenoids and other non-chlorophyll pigments that participate in photosynthesis are referred to as accessory pigments. Their structures and properties vary among different species (Figure 2). Figure 2. Accessory pigments. Two examples of pigments that may assist chlorophyll in photosynthetic light absorption. The photosynthetic pigments are contained in the membranes of subcellular organelles called chloroplasts (Figure 3), which reside inside leaf cells. The interior air spaces of the leaf are connected to the outside atmosphere through tiny pores called stomata, which allow for the inward movement of CO2 and the outward movement of O2. The double membrane surrounding the chloroplast is permeable to small uncharged molecules such as CO2 and O2, thereby allowing for exchange of these gases during photosynthesis. The pigment-containing membranes inside the chloroplast are referred to as thylakoids, and are arranged in stacks called grana, which are strikingly similar to the arrangement of disks in the photoreceptor cells of the vertebrate retina. This arrangement of stacked membranes has apparently evolved independently several times as an extremely efficient system for the interception and capture of light energy.

The chlorophyll and carotenoid molecules in the thylakoid membranes are not free, but exist in the form of pigment-protein complexes, sometimes referred to as "light-harvesting complexes." These proteins maintain the pigments in the proper orientation for efficient energy transfer, and also bring about slight changes in the wavelengths of light they can absorb. The non-membranous region inside the chloroplast is called the stroma. It contains an aqueous solution of enzymes and various metabolites that participate in the reduction of CO2 to carbohydrate via the Calvin Cycle, as described below. Figure 3. The chloroplast. Photosynthesis occurs in the chloroplasts of algae and higher plants. Capture of light energy takes place on the interior membrane surfaces, while reduction of CO2 to carbohydrate occurs in the stroma. Absorption of Photons and Transfer of Energy When a plant is exposed to light, photons of appropriate wavelength will strike and be absorbed by the pigment-protein complexes arrayed on the thylakoid membranes. When this happens, the energy of the photon is transferred to the pigment molecule, thus causing the pigment to go into an electronically excited state.

In other words, the energy originally associated with the photon is now contained within the pigment molecule. Once a pigment molecule has gone into an excited state, it is possible for the excitation energy to be transferred to an adjacent pigment molecule. Dispersed among the pigment-protein complexes on the thylakoid membranes are special structures called reaction centers. They are fewer in number than the light-harvesting complexes, and consist of proteins, chlorophyll, and several other pigments in a highly structured arrangement. This will be described later in a detailed module. There are two types of reaction centers, called Photosystem I (PSI or P700) and Photosystem II (PSII or P680). The array of pigment-protein complexes on the thylakoid membranes functions as an antenna for gathering light energy. Excitation energy is transferred from one chlorophyll to another until the energy reaches a reaction center (Figure 4). This causes one of the chlorophyll molecules within the reaction center to go into an excited state. Figure 4. An Antenna Complex. The array of pigment-protein molecules (e.g., chlorophyll) that absorbs light is referred to as an antenna complex.

In this simplified figure, energy is absorbed from a photon of light, and transferred stepwise to the reaction center, P700. In the reaction center, the energy is imparted to an electron, which is passed on to a chain of carriers. Formation of High Energy Chemical Compounds Two kinds of energy-containing molecules are formed in the light reactions of photosynthesis: adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide phosphate (NADPH), which serves as a source of reducing power. The PSI and PSII reaction centers are interconnected by a series of compounds that function as electron carriers (Figure 5). An excited electron in a PSII reaction center enters this chain of carriers and moves from one to the next until it reaches a PSI reaction center. This transport of an electron between the two types of reaction centers results in the pumping of hydrogen ions (H+) across the thylakoid membrane, thus forming a gradient with a high H+ concentration inside the thylakoid compartments and a relatively low concentration on the stroma side. The potential energy associated with this gradient is then used to form ATP by a mechanism similar to that by which ATP is generated in mitochondria. Figure 5.

Photosystems I and II. The relationship between PSI and PSII, the connecting carrier chain and the generation of high energy compounds is shown as an electron passes through the system, gaining and losing energy. P680 = reaction center of PSII; P700 = reaction center of PSI; PQ = plastoquinone; cytb6f = cytochrome c b6f; PC = plastocyanin; Fdx = ferredoxin. The next step in the process is that an excited electron in PSI is transferred to a molecule of NADP, along with an H+, thereby reducing it to NADPH. Notice that the electrons leaving PSI are being replaced by ones that came through the electron carriers from PSII. Finally, in order for this process to continue, the electrons that were removed from PSII have to be replaced. This is achieved by the splitting of an H2O molecule, which yields electrons, H+, and oxygen. The oxygen from two water molecules forms O2, which then passes through the stomata into the atmosphere. In summary, what has been achieved is the conversion of photon energy into chemical energy in the form of ATP and NADPH, with the concomitant formation of O2.

Reduction of Atmospheric CO2 to Carbohydrate The second major phase of photosynthesis involves the conversion of CO2 from the atmosphere into carbohydrates and other biological molecules. This set of reactions is sometimes referred to as the "dark reactions" of photosynthesis because light is not directly involved. However, this terminology is somewhat misleading because light is required for the formation of the ATP and NADPH needed for energy and reducing power. If a photosynthesizing plant was suddenly put in the dark, the so-called dark reactions would only continue until the supply of ATP and NADPH was depleted. The conversion of CO2 into carbohydrate takes place in the chloroplast stroma and follows a complex metabolic pathway called the Calvin Cycle or the Reductive Pentose Phosphate Pathway, each step of which is catalyzed by a specific enzyme. The details of this pathway will be dealt with in another module, but its overall form is shown in Figure 6. Carbon dioxide first combines with a five carbon compound called ribulose-1,5 -bisphosphate (RUBP), which then immediately splits into two three carbon compounds.

Using energy and reducing power from ATP and NADPH, these are converted into 3-phosphoglyceraldehyde (3PGAL), which can be used by the plant to manufacture carbohydrates and various other biological molecules and to regenerate RUBP. Figure 6. The Calvin Cycle. In this series of reactions, only a few of which are shown here, carbon dioxide from the atmosphere is converted into carbohydrates. Understanding this cycle earned a Nobel Prize for Melvin Calvin in 1961. In order for the Calvin Cycle to continue, there must be a continuous supply of RUBP. Thus not all of the 3PGAL produced can go into the synthesis of carbohydrates - a significant portion (about 5 molecules out of 6) must be used to make fresh RUBP. This also requires ATP coming from the light reactions, and is what gives the pathway its cyclic nature. In summary, ATP and NADPH formed in the light reactions of photosynthesis are used to convert atmospheric CO2 into carbohydrates, which represent a stable, long term form of energy storage. Overall, energy that originated as photons from the sun is now contained in carbohydrates and other molecules that serve as food for the plant or for an animal that eats the plant.

Environmental Aspects of Photosynthesis Because of the fundamental biological significance of photosynthesis and its importance in agricultural productivity, a considerable amount of research has been directed toward understanding the effects that environmental factors such as light, temperature, rainfall, salinity, and disease have on the process. In recent years, the effects of elevated atmospheric CO2 and of increased ultraviolet radiation exposure due to depletion of the Earth's ozone layer have received considerable attention. The potential for forests and other photosynthetic ecosystems to mitigate increasing atmospheric CO2 is also an active area of research. Different species, and in some cases even different cultivars of the same species, vary widely in their ability to tolerate the multitude of environmental stresses that a photosynthesizing plant may encounter. The natural geographic distribution of plant species can be understood in terms of their ability to photosynthesize efficiently under the range of conditions that they encounter in the Earth's various ecosystems. Likewise, environmental factors govern the range over which specific agricultural crops can be successfully introduced and cultivated. Both traditional plant breeding methods and the techniques of modern molecular biology are being used to produce plants that can maintain their vigor and sustain high rates of photosynthesis when subjected to environmental stress.

These approaches have been very successful in the past and promise to further increase crop productivity and decrease the need for fertilizers, herbicides, and insecticides. These topics will be dealt with in more detail in another module. Suggested Readings Blankenship, R. E. 2002. Molecular Mechanisms of Photosynthesis. Blackwell Science Publishers, Oxford, UK, Malden, MA, USA. Jansson, J. 1994. The Light-Harvesting Chlorophyll a/b-Binding Proteins. Biochimica Biophysica Acta, 1184: 1919. Long, S. P., E. A. Ainsworth, A. Rogers, and D. R. Ort. 2004. Rising Atmospheric Carbon Dioxide: Plants FACE the Future. Annual Review of Plant Biology, 55: 591-628. Nelson, N. and C. F. Yocum. 2006. Structure and Function of Photosystems I and II. Annual Review of Plant Biology, 57: 521-565. Raghavendra, A.

S., ed. 1998. Photosynthesis: A Comprehensive Treatise. Cambridge University Press, Cambridge, UK, New York, USA. Raven, P. H., G. B. Johnson, J. Losos, and S. Singer. 2005. Biology, 7th. ed. McGraw-Hill, New York. Szalai, V. A. and G. W. Brudvig. 1998. How Plants Produce Dioxygen. American Scientist, 86: 542-551. Taiz, L. and E. Zeiger. 2006. Plant Physiology, 4th. ed. Sinauer Associates, Sunderland, MA. Wild, A. and R. Ball. 1997. Photosynthetic Unit and Photosystems: History of Research and Current View. Backhuys Publishers, Leiden. 3/25/08 photosynthesis (chemistry) Photosynthesis requires the green pigment CHLOROPHYLL, which absorbs sunlight. Turns out there are 2 types of chlorophyll, chlorophyll a & chlorophyll b.

They are slightly different chemically, but similar in that they like to absorb blue, violet, orange, & red wavelengths of light, & tend to REFLECT (not absorb) yellow & green wavelengths. So what? Well, plants can't use light energy that they reflect. So, since plants reflect yellows & greens, that energy cannot be used for photosynthesis. Translation? Plants under yellow & green light will carry out photosynthesis at a lower rate than the same type of plant under other colors (blue, violet, red). (This idea makes a nifty science project.) Detail #3 The chemical reaction of photosynthesis (as I hinted at in Detail #1) occur in two phases. Phase #1 is called "photolysis" or the "photochemical reactions" or (if you don't like those two names) the "light reactions". These reactions (whatever you call them) require light (duh). Phase #2 is called "carbon fixation" or the "dark reactions". These reactions do not require light. Remember the parts of a chloroplast? The light reactions occur in the grana, which contain the chlorophyll for absorbing light. The dark reactions occur in the stroma.

(see pic above) Brain Refresher Before we get into more details, please recall that the overall chemical equation for photosynthsis is the following: 6CO2 + 12H2O + light energy ---> C6H12O6 + 6O2 + 6H2O or, in words: carbon dioixde + water + light energy ---> glucose + oxygen + water OK, onward .... #4 Detailed details about the light reactions. Of the substances in the overall equation, the light reactions use light & water, & produce the oxygen "waste product". Remember that name "photolysis"? That translates as light (photo-) splits (-lysis). Turns out that the light energy is used to split the water molecule, which produces H+ & O- ions. The oxygen is released from the plant into the air. ATP & NADPH2 are energy molecules. They will be used up during the dark reactions, as will the H+ ions.

the light reactions Upon close inspection of the photosynthesis equation, 6CO2 + 12H2O + light energy ---> C6H12O6 + 6O2 + 6H2O one may notice that two reactants (CO2 & H2O) contain oxygen. So, the skeptical folks out there would ask, "How do you know that the oxygen we breathe comes from the water molecule that is split in the light reactions & NOT from the CO2?" The answer: Scientists have used an isotope of oxygen called O-18 to follow oxygen atoms through the chemical maze of photosynthesis. When they tagged the "O" of the CO2 none of the oxygen being released from the plant contained O-18. But when they tagged the "O" in the H2O, all of the oxygen being produced contained O-18. So the O we breathe originates from water molecules used by the plant during photosynthesis. Detail #5 Don't be afraid of the dark ... reactions, a.k.a. "carbon fixation".

These reactions use the energy of the ATP & NADPH2 (produced in the light reactions) to combine the H+ (also produced in the light reactions) with CO2. Now the end product we're after here is glucose. To build glucose we need C, H, & O, which we have in our H+ & CO2. Nifty, huh? So there's a bunch of reactions from which & a 3-carbon compound called PGAL is produced. Think of PGAL as 1/2 of a glucose. Then, 2 PGAL's are combined & WA-LA! A glucose is born! Simple 7. Atmospheric oxygen that is inhaled by animals comes from: a) carbon dioxide molecules split during the light reactions b) carbon dioxide split during the dark reactions c) water molecules split during the light reactions d) water molecules split during the dark reactions 8. What change occurs during photosynthesis? a) solar energy is converted to chemical energy b) kinetic energy is converted to chemical energy c) chemical energy is converted to radiant energy d) water is converted to chemical energy For questions #9-18 choose the best description from the list.

A - photochemical reactions only B - carbon fixation reactions only C - both the photochemical & carbon fixation reactions D - neither the photochemical & carbon fixation reactions 9. Water molecules are split. 10. Carbon dioxide is used. 11. PGAL is formed. d) lactic acid - in sore muscles do to anaerobic respiration 6. In what part of photosynthesis would the tracer carbon-14 be used to study the path of carbon through the chemical reactions? a) the absorption of light by chlorophyll b) the dark reactions - involve carbon dioxide c) photolysis (note that c & d are the same thing so neither is right) 7. Atmospheric oxygen that is inhaled by animals comes from: a) carbon dioxide molecules split during the light reactions b) carbon dioxide split during the dark reactions c) water molecules split during the light reactions d) water molecules split during the dark reactions 8. What change occurs during photosynthesis?

a) solar energy is converted to chemical energy - sunlight --> food energy b) kinetic energy is converted to chemical energy c) chemical energy is converted to radiant energy d) water is converted to chemical energy For questions #9-18 choose the best description from the list. A - photochemical reactions only B - carbon fixation reactions only C - both the photochemical & carbon fixation reactions D - neither the photochemical & carbon fixation reactions 9. Water molecules are split. A 10. Carbon dioxide is used. B - carbon fixation, carbon dioxide 11. PGAL is formed. B 12. Light energy is absorbed by chlorophyll. A - "photo" for light 13. The reactions occur in the stroma of the chloroplast. B 14. The reactions occur in the grana of the chloroplast. A 15. Glucose is formed. B - last product, formed from PGAL 16. Enzymes are present. C - all reactions require enzymes 17. Oxygen is released. A - comes from the split water 18. Oxygen gas is used.

D - oxygen is a product, not a reactant Photolysis and the Light Reactions: Definitions, Steps, Reactants & Products - Video & Lesson Transcript | Study.com Photolysis and the Light Reactions: Definitions, Steps, Reactants & Products Watch short & fun videos Start Your Free Trial Today An error occurred trying to load this video. Try refreshing the page, or contact customer support. You must create an account to continue watching Register for a free trial Are you a student or a teacher? I am a student Start Your Free Trial To Continue Watching As a member, you'll also get unlimited access to over lessons in math, English, science, history, and more. Plus, get practice tests, quizzes, and personalized coaching to help you succeed. Coming up next: Dark Reactions of Photosynthesis: The Calvin-Benson Cycle You're on a roll. Keep up the good work! Your next lesson will play in 10 seconds 0:05 Photosynthesis Add to Add to Add to Want to watch this again later? Log in or sign up to add this lesson to a Custom Course. Custom Courses are courses that you create from Study.com lessons.

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Create an account to start this course today Try it free for 5 days! Instructor: Kristin Klucevsek Kristin has taught college Biology courses and has her doctorate in Biology. In this lesson, we'll learn how electrons get excited during the light-dependent reactions of photosynthesis, jumping off photosystem reaction centers like they were trampolines! In addition, we'll learn how the splitting of water creates reactants for this part of photosynthesis while creating a little fresh air for us. Photosynthesis When plants start to get a little hungry, they need to only turn their leaves to the sun to start photosynthesis , the process that turns energy from sunlight into food. In this lesson, we'll start to break down how this process takes place. Photosynthesis consists of two parts. The first part of this process consists of the light reactions, while the second is called the dark reactions. This might seem like two sides of something like Star Wars' good versus evil, but we promise that everything is good when it comes to photosynthesis. It's just that the light-dependent reactions use sunlight, and the dark reactions don't use sunlight.

Before we begin on how the light reactions work, let's remind ourselves of the products and reactants for photosynthesis. In this process, remember that light energy, carbon dioxide, and water are used to produce glucose, or sugar, and oxygen. In this lesson, we'll learn how two of these reactants are used. Specifically, we'll learn how light energy is used to split water, a process known as photolysis. Photosystems Inside a plant cell, inside a chloroplast, and within a thylakoid membrane, there are photosystems, which are the sites of the light-dependent photosynthesis reactions. There are two types of photosystems: photosystem I and photosystem II. Photosystem II is actually used before photosystem I, but they are numbered for the order they were discovered. Light energy is absorbed at the reaction center of photosystems Photosystems contain an antenna complex. The antenna complex contains the pigment chlorophyll a, and it's what makes them green. Photosystems also have accessory pigments of other colors. Pigments, you'll remember, absorb specific wavelengths of light.

In photosystem II, chlorophyll molecules are called P680 because they optimally absorb light at this wavelength, while in photosystem I, they are P700. In these photosystems, pigments pass the energy absorbed from light to the reaction center - this is in the middle of the photosystem. The reaction center contains a special pair of chlorophyll molecules coupled with proteins. Here comes the exciting part, literally. When the light energy hits the reaction center, electrons from the chlorophyll molecules are excited to a higher energy level. Imagine light energy jumping on a trampoline along a photosystem and then boom, landing straight in the center, sending electrons up in the air, handed to a primary electron acceptor! Now, what goes up must come back down. These energized electrons come back down in energy levels by moving through an electron transport chain. You might remember the electron transport chain from cellular respiration. The electron transport chain in photosynthesis is similar to this. Electron acceptor proteins reside within the thylakoid membrane. These proteins also become reduced as they accept electrons and oxidized as they pass electrons down this chain.

The primary electron acceptor passes the electrons down to these membrane proteins as they decrease in energy level until they reach photosystem I. Meanwhile, the electron transport chain of photosynthesis, just like in cellular respiration, creates a proton concentration gradient that makes ATP, or chemical energy, as protons move through an ATP synthase. Photolysis When the electrons leave the chlorophyll molecules, it leaves behind a 'hole.' This electron hole is filled in by a water molecule that is oxidized, or loses electrons, as it essentially splits into two hydrogen atoms, or protons, and an oxygen atom. Two oxygen atoms combine to form one molecule of oxygen gas that gets released as a waste. Of course, this is waste to the plant, but it's gold to us, and we couldn't live without it! Meanwhile, the new electrons from this split of a water molecule are next in line to be bounced up to the primary electron acceptor. The protons that are released help create the proton gradient in the thylakoid lumen important to making ATP. × PHOTOSYNTHESIS PHOTOSYNTHESIS What is Photosynthesis?

| Back to Top Photosynthesis is the process by which plants, some bacteria, and some protistans use the energy from sunlight to produce sugar, which cellular respiration converts into ATP , the "fuel" used by all living things. The conversion of unusable sunlight energy into usable chemical energy, is associated with the actions of the green pigment chlorophyll . Most of the time, the photosynthetic process uses water and releases the oxygen that we absolutely must have to stay alive. Oh yes, we need the food as well! We can write the overall reaction of this process as: 6H2O + 6CO2 ----------> C6H12O6+ 6O2 Most of us don't speak chemicalese, so the above chemical equation translates as: six molecules of water plus six molecules of carbon dioxide produce one molecule of sugar plus six molecules of oxygen Diagram of a typical plant, showing the inputs and outputs of the photosynthetic process. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission.

Leaves and Leaf Structure | Back to Top Plants are the only photosynthetic organisms to have leaves (and not all plants have leaves). A leaf may be viewed as a solar collector crammed full of photosynthetic cells. The raw materials of photosynthesis, water and carbon dioxide, enter the cells of the leaf, and the products of photosynthesis, sugar and oxygen, leave the leaf. Cross section of a leaf, showing the anatomical features important to the study of photosynthesis: stoma, guard cell, mesophyll cells, and vein. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Water enters the root and is transported up to the leaves through specialized plant cells known as xylem (pronounces zigh-lem). Land plants must guard against drying out (desiccation) and so have evolved specialized structures known as stomata to allow gas to enter and leave the leaf. Carbon dioxide cannot pass through the protective waxy layer covering the leaf (cuticle), but it can enter the leaf through an opening (the stoma; plural = stomata; Greek for hole) flanked by two guard cells.

Likewise, oxygen produced during photosynthesis can only pass out of the leaf through the opened stomata. Unfortunately for the plant, while these gases are moving between the inside and outside of the leaf, a great deal water is also lost. Cottonwood trees, for example, will lose 100 gallons of water per hour during hot desert days. Carbon dioxide enters single-celled and aquatic autotrophs through no specialized structures. Pea Leaf Stoma, Vicea sp. (SEM x3,520). This image is copyright Dennis Kunkel at , used with permission. The Nature of Light | Back to Top White light is separated into the different colors (=wavelengths) of light by passing it through a prism. Wavelength is defined as the distance from peak to peak (or trough to trough). The energy of is inversely porportional to the wavelength: longer wavelengths have less energy than do shorter ones. Wavelength and other saspects of the wave nature of light. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The order of colors is determined by the wavelength of light.

Visible light is one small part of the electromagnetic spectrum. The longer the wavelength of visible light, the more red the color. Likewise the shorter wavelengths are towards the violet side of the spectrum. Wavelengths longer than red are referred to as infrared, while those shorter than violet are ultraviolet. The electromagnetic spectrum. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Light behaves both as a wave and a particle. Wave properties of light include the bending of the wave path when passing from one material (medium) into another (i.e. the prism, rainbows, pencil in a glass-of-water, etc.). The particle properties are demonstrated by the photoelectric effect. Zinc exposed to ultraviolet light becomes positively charged because light energy forces electrons from the zinc. These electrons can create an electrical current. Sodium, potassium and selenium have critical wavelengths in the visible light range. The critical wavelength is the maximum wavelength of light (visible or invisible) that creates a photoelectric effect. Chlorophyll and Accessory Pigments | Back to Top A pigment is any substance that absorbs light.

The color of the pigment comes from the wavelengths of light reflected (in other words, those not absorbed). Chlorophyll , the green pigment common to all photosynthetic cells, absorbs all wavelengths of visible light except green, which it reflects to be detected by our eyes. Black pigments absorb all of the wavelengths that strike them. White pigments/lighter colors reflect all or almost all of the energy striking them. Pigments have their own characteristic absorption spectra, the absorption pattern of a given pigment. Absorption and transmission of different wavelengths of light by a hypothetical pigment. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Chlorophyll is a complex molecule. Several modifications of chlorophyll occur among plants and other photosynthetic organisms. All photosynthetic organisms (plants, certain protistans, prochlorobacteria, and cyanobacteria) have chlorophyll a . Accessory pigments absorb energy that chlorophyll a does not absorb.

Accessory pigments include chlorophyll b (also c, d, and e in algae and protistans), xanthophylls, and carotenoids (such as beta-carotene). Chlorophyll a absorbs its energy from the Violet-Blue and Reddish orange-Red wavelengths, and little from the intermediate (Green-Yellow-Orange) wavelengths. Molecular model of chlorophyll. The above image is from . Molecular model of carotene. The above image is from . Carotenoids and chlorophyll b absorb some of the energy in the green wavelength. Why not so much in the orange and yellow wavelengths? Both chlorophylls also absorb in the orange-red end of the spectrum (with longer wavelengths and lower energy). The origins of photosynthetic organisms in the sea may account for this. Shorter wavelengths (with more energy) do not penetrate much below 5 meters deep in sea water. The ability to absorb some energy from the longer (hence more penetrating) wavelengths might have been an advantage to early photosynthetic algae that were not able to be in the upper ( photic) zone of the sea all the time. The molecular structure of chlorophylls.

Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The action spectrum of photosynthesis is the relative effectiveness of different wavelengths of light at generating electrons. If a pigment absorbs light energy, one of three things will occur. Energy is dissipated as heat. The energy may be emitted immediately as a longer wavelength, a phenomenon known as fluorescence. Energy may trigger a chemical reaction, as in photosynthesis. Chlorophyll only triggers a chemical reaction when it is associated with proteins embedded in a membrane (as in a chloroplast) or the membrane infoldings found in photosynthetic prokaryotes such as cyanobacteria and prochlorobacteria. Absorption spectrum of several plant pigments (left) and action spectrum of elodea (right), a common aquarium plant used in lab experiments about photosynthesis. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The structure of the chloroplast and photosynthetic membranes | Back to Top The thylakoid is the structural unit of photosynthesis.

Both photosynthetic prokaryotes and eukaryotes have these flattened sacs/vesicles containing photosynthetic chemicals. Only eukaryotes have chloroplasts with a surrounding membrane. Thylakoids are stacked like pancakes in stacks known collectively as grana . The areas between grana are referred to as stroma . While the mitochondrion has two membrane systems, the chloroplast has three, forming three compartments. Structure of a chloroplast. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Stages of Photosynthesis | Back to Top Photosynthesis is a two stage process. The first process is the Light Dependent Process ( Light Reactions ), requires the direct energy of light to make energy carrier molecules that are used in the second process. The Light Independent Process (or Dark Reactions ) occurs when the products of the Light Reaction are used to form C-C covalent bonds of carbohydrates. The Dark Reactions can usually occur in the dark, if the energy carriers from the light process are present.

Recent evidence suggests that a major enzyme of the Dark Reaction is indirectly stimulated by light, thus the term Dark Reaction is somewhat of a misnomer. The Light Reactions occur in the grana and the Dark Reactions take place in the stroma of the chloroplasts. Overview of the two steps in the photosynthesis process. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Light Reactions | Back to Top In the Light Dependent Processes (Light Reactions) light strikes chlorophyll a in such a way as to excite electrons to a higher energy state. In a series of reactions the energy is converted (along an electron transport process) into ATP and NADPH . Water is split in the process, releasing oxygen as a by-product of the reaction. The ATP and NADPH are used to make C-C bonds in the Light Independent Process (Dark Reactions). In the Light Independent Process, carbon dioxide from the atmosphere (or water for aquatic/marine organisms) is captured and modified by the addition of Hydrogen to form carbohydrates (general formula of carbohydrates is [CH2O]n).

The incorporation of carbon dioxide into organic compounds is known as carbon fixation. The energy for this comes from the first phase of the photosynthetic process. Living systems cannot directly utilize light energy, but can, through a complicated series of reactions, convert it into C-C bond energy that can be released by glycolysis and other metabolic processes. Photosystems are arrangements of chlorophyll and other pigments packed into thylakoids. Many Prokaryotes have only one photosystem, Photosystem II (so numbered because, while it was most likely the first to evolve, it was the second one discovered). Eukaryotes have Photosystem II plus Photosystem I. Photosystem I uses chlorophyll a, in the form referred to as P700. Photosystem II uses a form of chlorophyll a known as P680. Both "active" forms of chlorophyll a function in photosynthesis due to their association with proteins in the thylakoid membrane. Action of a photosystem. This image is from the University of Minnesota page at . Photophosphorylation is the process of converting energy from a light-excited electron into the pyrophosphate bond of an ADP molecule.

This occurs when the electrons from water are excited by the light in the presence of P680. The energy transfer is similar to the chemiosmotic electron transport occurring in the mitochondria. Light energy causes the removal of an electron from a molecule of P680 that is part of Photosystem II. The P680 requires an electron, which is taken from a water molecule, breaking the water into H+ ions and O-2 ions. These O-2 ions combine to form the diatomic O2 that is released. The electron is "boosted" to a higher energy state and attached to a primary electron acceptor, which begins a series of redox reactions, passing the electron through a series of electron carriers, eventually attaching it to a molecule in Photosystem I. Light acts on a molecule of P700 in Photosystem I, causing an electron to be "boosted" to a still higher potential. The electron is attached to a different primary electron acceptor (that is a different molecule from the one associated with Photosystem II).

The electron is passed again through a series of redox reactions, eventually being attached to NADP+ and H+ to form NADPH, an energy carrier needed in the Light Independent Reaction. The electron from Photosystem II replaces the excited electron in the P700 molecule. There is thus a continuous flow of electrons from water to NADPH. This energy is used in Carbon Fixation. Cyclic Electron Flow occurs in some eukaryotes and primitive photosynthetic bacteria. No NADPH is produced, only ATP. This occurs when cells may require additional ATP, or when there is no NADP+ to reduce to NADPH. In Photosystem II, the pumping to H ions into the thylakoid and the conversion of ADP + P into ATP is driven by electron gradients established in the thylakoid membrane. Noncyclic photophosphorylation (top) and cyclic photophosphorylation (bottom). These processes are better known as the light reactions. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission.

The above diagrams present the "old" view of photophosphorylation. We now know where the process occurs in the chloroplast, and can link that to chemiosmotic synthesis of ATP. Chemiosmosis as it operates in photophosphorylation within a chloroplast. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. Halobacteria, which grow in extremely salty water, are facultative aerobes, they can grow when oxygen is absent. Purple pigments, known as retinal (a pigment also found in the human eye) act similar to chlorophyll. The complex of retinal and membrane proteins is known as bacteriorhodopsin, which generates electrons which establish a proton gradient that powers an ADP-ATP pump, generating ATP from sunlight without chlorophyll. This supports the theory that chemiosmotic processes are universal in their ability to generate ATP. Dark Reaction | Back to Top Carbon-Fixing Reactions are also known as the Dark Reactions (or Light Independent Reactions).

Carbon dioxide enters single-celled and aquatic autotrophs through no specialized structures, diffusing into the cells. Land plants must guard against drying out (desiccation) and so have evolved specialized structures known as stomata to allow gas to enter and leave the leaf. The Calvin Cycle occurs in the stroma of chloroplasts (where would it occur in a prokaryote?). Carbon dioxide is captured by the chemical ribulose biphosphate (RuBP ). RuBP is a 5-C chemical. Six molecules of carbon dioxide enter the Calvin Cycle, eventually producing one molecule of glucose. The reactions in this process were worked out by Melvin Calvin (shown below). The above image is from , Ernest OrlandoLawrence Berkeley National Laboratory. " One of the new areas, cultivated both in Donner and the Old Radiation Laboratory, was the study of organic compounds labeled with carbon-14. Melvin Calvin took charge of this work at the end of the war in order to provide raw materials for John Lawrence's researches and for his own study of photosynthesis.

Using carbon-14, available in plenty from Hanford reactors, and the new techniques of ion exchange, paper chromatography, and radioautography, Calvin and his many associates mapped the complete path of carbon in photosynthesis. The accomplishment brought him the Nobel prize in chemistry in 1961. (The preceding information was excerpted from the text of the Fall 1981 issue of LBL Newsmagazine.) Citation Caption: LBL News, Vol.6, No.3, Fall 1981 Melvin Calvin shown with some of the apparatus he used to study the role of carbon in photosynthesis." The first steps in the Calvin ccycle. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The first stable product of the Calvin Cycle is phosphoglycerate (PGA) , a 3-C chemical. The energy from ATP and NADPH energy carriers generated by the photosystems is used to attach phosphates to ( phosphorylate ) the PGA.

Eventually there are 12 molecules of glyceraldehyde phosphate (also known as phosphoglyceraldehyde or PGAL , a 3-C), two of which are removed from the cycle to make a glucose . The remaining PGAL molecules are converted by ATP energy to reform 6 RuBP molecules, and thus start the cycle again. Remember the complexity of life, each reaction in this process, as in Kreb's Cycle, is catalyzed by a different reaction-specific enzyme . C-4 Pathway | Back to Top Some plants have developed a preliminary step to the Calvin Cycle (which is also referred to as a C-3 pathway), this preamble step is known as C-4. While most C-fixation begins with RuBP, C-4 begins with a new molecule, phosphoenolpyruvate (PEP), a 3-C chemical that is converted into oxaloacetic acid (OAA, a 4-C chemical) when carbon dioxide is combined with PEP. The OAA is converted to Malic Acid and then transported from the mesophyll cell into the bundle-sheath cell, where OAA is broken down into PEP plus carbon dioxide.

The carbon dioxide then enters the Calvin Cycle, with PEP returning to the mesophyll cell. The resulting sugars are now adjacent to the leaf veins and can readily be transported throughout the plant. C-4 photosynthsis involves the separation of carbon fixation and carbohydrate systhesis in space and time. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The capture of carbon dioxide by PEP is mediated by the enzyme PEP carboxylase, which has a stronger affinity for carbon dioxide than does RuBP carboxylase When carbon dioxide levels decline below the threshold for RuBP carboxylase, RuBP is catalyzed with oxygen instead of carbon dioxide. The product of that reaction forms glycolic acid, a chemical that can be broken down by photorespiration, producing neither NADH nor ATP, in effect dismantling the Calvin Cycle. C-4 plants, which often grow close together, have had to adjust to decreased levels of carbon dioxide by artificially raising the carbon dioxide concentration in certain cells to prevent photorespiration.

C-4 plants evolved in the tropics and are adapted to higher temperatures than are the C-3 plants found at higher latitudes. Common C-4 plants include crabgrass, corn, and sugar cane. Note that OAA and Malic Acid also have functions in other processes, thus the chemicals would have been present in all plants, leading scientists to hypothesize that C-4 mechanisms evolved several times independently in response to a similar environmental condition, a type of evolution known as convergent evolution. Photorespiration. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. We can see anatomical differences between C3 and C4 leaves. Leaf anatomy of a C3 (top) and C4 (bottom) plant. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates ( ) and WH Freeman ( ), used with permission. The Carbon Cycle | Back to Top Plants may be viewed as carbon sinks , removing carbon dioxide from the atmosphere and oceans by fixing it into organic chemicals.

Plants also produce some carbon dioxide by their respiration, but this is quickly used by photosynthesis. Plants also convert energy from light into chemical energy of C-C covalent bonds. Animals are carbon dioxide producers that derive their energy from carbohydrates and other chemicals produced by plants by the process of photosynthesis. The balance between the plant carbon dioxide removal and animal carbon dioxide generation is equalized also by the formation of carbonates in the oceans. This removes excess carbon dioxide from the air and water (both of which are in equilibrium with regard to carbon dioxide). Fossil fuels, such as petroleum and coal, as well as more recent fuels such as peat and wood generate carbon dioxide when burned. Fossil fuels are formed ultimately by organic processes, and represent also a tremendous carbon sink. Human activity has greatly increased the concentration of carbon dioxide in air. This increase has led to global warming, an increase in temperatures around the world, the Greenhouse Effect. The increase in carbon dioxide and other pollutants in the air has also led to acid rain , where water falls through polluted air and chemically combines with carbon dioxide, nitrous oxides, and sulfur oxides, producing rainfall with pH as low as 4.

This results in fish kills and changes in soil pH which can alter the natural vegetation and uses of the land. The Global Warming problem can lead to melting of the ice caps in Greenland and Antarctica, raising sea-level as much as 120 meters. Changes in sea-level and temperature would affect climate changes, altering belts of grain production and rainfall patterns. Learning Objectives | Back to Top After completing this chapter you should be able to: Study the general equation for photosynthesis and be able to indicate in which process each reactant is used and each product is produced. List the two major processes of photosynthesis and state what occurs in those sets of reactions. Distinguish between organisms known as autotrophs and those known as heterotrophs as pertains to their modes of nutrition. Explain the significance of the ATP/ADP cycle. Describe the nature of light and how it is associated with the release of electrons from a photosystem. Describe how the pigments found on thylakoid membranes are organized into photosystems and how they relate to photon light energy. Describe the role that chlorophylls and the other pigments found in chloroplasts play to initiate the light-dependent reactions.

Describe the function of electron transport systems in the thylakoid membrane. Explain the role of the two energy-carrying molecules produced in the light-dependent reactions (ATP and NADPH) in the light-independent reactions. Describe the Calvin-Benson cycle in terms of its reactants and products. Explain how C-4 photosynthesis provides an advantage for plants in certain environments. Describe the phenomenon of acid rain, and how photosynthesis relates to acid rain and the carbon cycle.. Review Questions | Back to Top 1. The organic molecule produced directly by photosynthesis is: a) lipids; b) sugar; c) amino acids; d) DNA 2. The photosynthetic process removes ___ from the environment. a) water; b) sugar; c) oxygen; d) chlorophyll; e) carbon dioxide 3. The process of splitting water to release hydrogens and electrons occurs during the _____ process. a) light dependent; b) light independent; c) carbon fixation; d) carbon photophosphorylation; e) glycolysis 4. The process of fixing carbon dioxide into carbohydrates occurs in the ____ process.

a) light dependent; b) light independent; c) ATP synthesis; d) carbon photophosphorylation; e) glycolysis 5. Carbon dioxide enters the leaf through ____. a) chloroplasts; b) stomata: c) cuticle; d) mesophyll cells; e) leaf veins 6. The cellular transport process by which carbon dioxide enters a leaf (and by which water vapor and oxygen exit) is ___. a) osmosis; b) active transport; c. co- transport; d) diffusion; e) bulk flow 7. Which of the following creatures would not be an autotroph? a) cactus; b) cyanobacteria; c) fish; d) palm tree; e) phytoplankton 8. The process by which most of the world's autotrophs make their food is known as ____. a) glycolysis; b) photosynthesis; c) chemosynthesis; d) herbivory; e) C-4 cycle 9. The process of ___ is how ADP + P are converted into ATP during the Light dependent process.

a) glycolysis; b) Calvin Cycle; c) chemiosmosis; d) substrate-level phosphorylation; e) Kreb's Cycle 10. Once ATP is converted into ADP + P, it must be ____. a) disassembled into components (sugar, base, phosphates) and then ressembled; b) recharged by chemiosmosis; c) converted into NADPH; d) processed by the glycolysis process; e) converted from matter into energy. 11. Generally speaking, the longer the wavelenght of light, the ___ the available energy of that light. a) smaller; b) greater; c) same 12. The section of the electromagnetic spectrum used for photosynthesis is ___. a) infrared; b) ultraviolet; c) x-ray; d) visible light; e) none of the above 13. The colors of light in the visible range (from longest wavelength to shortest) is ___. a) ROYGBIV; b) VIBGYOR; c) GRBIYV; d) ROYROGERS; e) EBGDF 14.

The photosynthetic pigment that is essential for the process to occur is ___. a) chlorophyll a; b) chlorophyll b; c) beta carotene; d) xanthocyanin; e) fucoxanthin 15. When a pigment reflects red light, _____. a) all colors of light are absorbed; b) all col;ors of light are reflected; c) green light is reflected, all others are absorbed; d) red light is reflected, all others are absorbed; e) red light is absorbed after it is reflected into the internal pigment molecules. 16. Chlorophyll a absorbs light energy in the ____color range. a) yellow-green; b) red-organge; c) blue violet; d) a and b; e) b and c. 17. A photosystem is ___. a) a collection of hydrogen-pumping proteins; b) a collection of photosynthetic pigments arranged in a thylakjoid membrane; c) a series of electron-accepting proteins arranged in the thylakoid membrane; d.

found only in prokaryotic organisms; e) multiple copies of chlorophyll a located in the stroma of the chloroplast. 18. The individual flattened stacks of membrane material inside the chloroplast are known as ___. a) grana; b) stroma; c) thylakoids; d) cristae; e) matrix 19. The fluid-filled area of the chloroplast is the ___. a) grana; b) stroma; c) thylakoids; d) cristae; e) matrix 20. The chloroplast contains all of these except ___. a) grana; b) stroma; c) DNA; d) membranes; e) endoplasmic reticulum 21. The chloroplasts of plants are most close in size to __. a) unfertilized human eggs; b) human cheek cells; c) human nerve cells; d) bacteria in the human mouth; e) viruses 22. Which of these photosynthetic organisms does not have a chloroplast? a) plants; b) red algae; c) cyanobacteria; d) diatoms; e) dinoflagellates 23.

The photoelectric effect refers to ____. a) emission of electrons from a metal when energy of a critical wavelength strikes the metal; b) absorbtion of electrons from the surrounding environment when energy of a critical wavelength is nearby; c) emission of electrons from a metal when struck by any wavelength of light; d) emission of electrons stored in the daytime when stomata are open at night; e) release of NADPH and ATP energy during the Calvin Cycvle when light iof a specific wavelength strikes the cell. 24. Light of the green wavelengths is commonly absorbed by which accessory pigment? a) chlorophyll a; b) chlorophyll b; c) phycocyanin; d) beta carotene 25. The function of the electron transport proteins in the thyakoid membranes is ___. a) production of ADP by chemiosmosis; b) production of NADPH by substrate-level phosphorylation; c) pumping of hydrogens into the thylakoid space for later generation of ATP by chemiosmosis; d) pumping of hydrogens into the inner cristae space for later generation of ATP by chemiosmosis; e) preparation of water for eventual incorporation into glucose 26.

ATP is known as the energy currency of the cell because ____. a) ATP is the most readily usable form of energy for cells; b) ATP passes energy along in an electron transport chain; c) ATP energy is passed to NADPH; d) ATP traps more energy than is produced in its formation; e) only eukaryotic cells use this energy currency. 27. Both cyclic and noncyclic photophosphorylation produce ATP. We can infer that the purpose of ATP in photosynthesis is to ____. a) supply hydrogen to the carbohydrate; b) supply carbon to the carbohydrate; c) supply energy that can be used to form a carbohydrate; d) transfer oxygens from the third phosphate group to the carbohydrate molecule; e) convert RuBP into PGA 28. The role of NADPH in oxygen-producing photosynthesis is to ____. a) supply hydrogen to the carbohydrate; b) supply carbon to the carbohydrate; c) supply energy that can be used to form a carbohydrate; d) transfer oxygens from the third phosphate group to the carbohydrate molecule; e) convert RuBP into PGA. 29. The dark reactions require all of these chemicals to proceed except ___.

a) ATP; b) NADPH; c) carbon dioxide; d) RUBP; e) oxygen 30. The first stable chemical formed by the Calvin Cycle is _____. a) RUBP; b) RU/18; c) PGA; d) PGAL; e) Rubisco 31. The hydrogen in the carbohydrate produced by the Calvin Cycle comes from ___ a.) ATP; b) NADPH; c) the environment if the pH is very acidic; d) a and b; e) a and c 32. The carbon incorporated into the carbohydrate comes from ___. a) ATP; b) NADPH; c) carbon dioxide; d) glucose; e) organic molecules 33. C-4 photosynthesis is so named because _____. a) it produces a three carbon compound as the first stable product of photosynthesis; b) it produces a four carbon compound as the first stable produc of photosynthesis; c) it produces four ATP and four NADPH molecules for carbon fixation.; d) there are only four steps in this form of carbon fixation into carbohydrate.

Question: Which step in photosynthesis is responsible for splitting water molecules?
Answer: Light reaction